The invention relates to a switching arrangement for converting analog signals, more particularly pulse amplitude modulation (PAM) signals, into digital signals, more particularly pulse code modulation (PCM) signals, and for converting the digital signals into analog signals. The invention has particular application in telecommunication installations having telephone stations, each having a transmitting unit providing analog signals and a receiving unit receiving analog signals.
A PCM scaling and coding circuit has become known (U.S. Pat. No. 3,594,765), wherein the analog signals to be coded are compared with a reference voltage varying in time and in the same direction, whereby during each comparison period a scaler counts from zero. If there is agreement between an analog signal to be coded and the reference voltage, the instantaneous counter position of the counter is stored; the counter position indicates the PCM word corresponding to the analog signal being coded.
In the switching arrangement of known construction the counter is further utilized for converting a PCM signal into an analog signal, whereby the counter is preset by the PCM signal to be converted in each case and then reset in a cyclic operation to its zero position. A signal occurring when this zero position is reached is used for defining the leading edge of a pulse-length-modulated signal. The trailing edge of such a pulse-length-modulated signal is defined by the end of the transmission cycle. The pulse-length-modulated signal is then converted into a PAM signal in a manner in itself know. Thus, the counter may be employed either during the conversion of an analog signal into a PCM signal or during the conversion of a PCM signal into a PAM signal. However, sometimes this takes a considerable amount of time.
Another switching arrangement is known for converting analog signals into PCM signals, using a non-linear characteristic and for converting PCM signals into analog signals, more particularly into PAM signals, using a non-linear characteristic (West German Unexamined Patent Application 2,333,299). In this known switching arrangement a comparison is made in an analog comparator, with a view to converting a PAM signal into a PCM signal, between a voltage varying with time along the non-linear characteristic and the PAM signal. If there is agreement between the PAM signal and the voltage varying with time, the analog comparator stops a memory circuit, which continually stores a code chain corresponding to the particular amplitude of the voltage involved. To convert the PCM signals into PAM signals, a voltage is used having an amplitude characteristic corresponding to the non-linear characteristic. To supply the voltages there is provided a single function generator which produces a voltage varying with time and corresponding to the non-linear characteristic, as far as its characteristic is concerned and which is activated by a clock-controlled counter for the delivery of the voltage. A code chain corresponding to the particular amplitude of the voltage involved is provided in the counter.
There is further connected to the counter a digital comparator which compares the PCM signals to be converted into PAM signals with all possible counter code chains and which in case of agreement between the particular PCM signal and one of said code chains causes the delivery as a PAM signal of the voltage of the function generator existing at the particular time. Although the switching arrangement just described can be utilized for the simultaneous conversion of a PAM signal into a PCM signal and of a PCM signal into a PAM signal, and although the counter provided for the switching arrangement concerned is provided as a single counter, the overall circuit complexity required is still excessive.
It is, therefore, an object of the invention to provide a relatively uncomplex circuit arrangement for converting in a simple manner analog signals into digital signals and digital signals into analog signals.